Visualization of the different intracranial vascular territories is usually obtained by using angiographic techniques. However, due to the difficulties to visualize the distal vascular bed with any angiographic technique it is difficult to infer the cortical borders of vascular territories. Arterial spin labeling (ASL) MRI provides information on cortical cerebral perfusion. Current techniques perform the labeling in each feeding artery as a separate experiment which is time-consuming. It would be desirable to find a way to acquire all vascular territory within the same experiment without loss of efficiency, and the disclosed invention is directed to this end.
MRI has been extensively developed to obtain angiographic information of vessels (MRA) and perfusion images in organs (1-5) (these numbers refer to the publications cited at the end of this disclosure; 11 publications are cited and their contents are hereby incorporated by reference herein as though fully set out.) The technique of arterial spin labeling (ASL) provides this type of information without the use of exogenous contrast agents but using magnetic labeling of blood spins. ASL is based on the idea that static tissue signal can be suppressed by acquisition of two data sets, which are prepared identically in the imaging region, while the region outside is prepared differently. After subtraction of those two data sets only spins are visible which experienced preparation outside the imaging slab and which were present in the imaging slab at time of acquisition.
The main application of this technique has been measurement of the global regional perfusion in the brain, but ASL is also capable of separating the regional perfusion signal in specific blood vessel territories (6-9). Common to all known published methods is that the blood of each feeding artery is labeled within a separate experiment. This leads to a long total measurement time if multiple vascular territories, such as left and right internal carotid artery (ICA) and basilar artery, need to be acquired (nine to fifteen minutes in (6,7)).
Arterial spin labeling also has been used to measure cerebral blood flow during rest and activation to obtain functional MRI (fMRI) images. (3, 12). As opposed to BOLD contrast, ASL contrast is not based on susceptibility and therefore, it should be possible to perform measurements in regions of high static field inhomogeneities close to tissue-air and tissue-bone interfaces.
To improve signal-to-noise ration (SNR) in ASL, many pairs of measurements (interleaved or consecutive images acquired with (labeled, L) and without (control, C) blood labeling are averaged then subtracted (L-C) to obtain cerebral blood flow or perfusion images. The static tissue in the data set can be suppressed using inversion pulses which tends to null brain static tissue without significantly affecting blood signal to further improve background suppression (13, 14, 17) and reduce the variation of background suppression in a time series.